Means for producing plutonium chain reactions



sa aa o aan 000Go E. P. wlGNER E1A| MEANS Foa PRonucING PLuToNIUM CHAIN REACTIONS Filed oct. s', 194e Jan. 24, 1961 MEANS Fon PRooUcING PLUroNIUM CHAIN RnAcrloNs Eugene P. Wigner and Alvin M. Weinberg, ont Riege, Tenn., assignors to the United States of America as represented by the United States Atomic Energy Commission Filed oct. s, 1946, ser. No. 101,911 z claims. (ci. zeit-193.2)

vtion wherein the average neutron energy is above about Another object of the invention is to reduce neutron absorption by neutronic impurities in a composition such as above described, by maintaining the average neutron energy at a sufficiently high value to avoid such absorption as hereinafter discussed.

A Vdifferent object of the invention is to Vsustain a chain reaction in a composition containing h sionable plutonium, the average neutron energy being above a value of about 0.3 e.v. at which maximum resonance absorption occurs in iissio'nable plutonium without fission thereof.

Another object of the invention is to provide in a composition such as above-described suti'icint neutron moderator to moderate the neutrons in said composition to energies substantially lower than fission energies, thereby substantially reducing the total amountof iiss onable material necessary to sustain the chain reaction, inasmuch as the fission cross section of such materfal is generally greater for neutrons at energy values lower than fission energies.

It has been found that in a neutronic reactor capable of sustaining a nuclear fission chain lreaction in an active portion containing neutron moderator and fissionable plutonium (Pu239), maximum resonance absorption of neutrons by the issionable plutonium without fission" thereof occurs for neutrons at an energy level of the order of 0.3 e.v., although' ity will Vbe understood that tissionable plutonium has many other ablorp'tonj teonarices of less significance.

fissionable plutonium, asV above defined, not only for the purpose of conserving neutrons" for the chain'rea'ction and thus increasing the neutron reproductfon ratio of the reactive composition but also in order to conserver valuable issio'nable material which by nuclear re-` ergy aboveo-Sfelv. kIt has 'bentound as statedabove.' tht'fiinxirnum'-resonance absorptionllor capture in Pom ratio of about 0.8 for neutrons at 0.13 evl asicornp'ared with 0.47 for neutrons of energieshilghe'i'oi lownthan It' is desirable, therefore,A to 'avoid maximum resonance absorption of neu'troris'by `incapable' of absorbing a substantial number of neu- 0.3 e.v. By providing a relatively small amount of moderator, such as water, most of the neutrons inthe systemycause fission of Pu2739 before being slowed to a value substantially less than 0.5 e.v., thus avoiding the above-mentioned maximum resonance absorption, In the embodment disclosed herein, Pu?3g is combined with neutron moderator in an atomic ratio wherebymost of the neutrons in the system cause fission of Pun?, before reaching' the above-mentioned `resonance energes therein, and it will tbe understood byv those skilfedjn the art that other iissionable isotopes, Vpreferably of high neutronic purity, may be substitutedfor Pu239.

It is known that a self-,sustaining nuclear iissfon chain reaction canV be obtained in neutronic reactorsV utilizing natural uranium, as a result of slow neutron fission of the U35 content of the natural uranium., In such reactors, discrete bodies of natural uranium of hfgh neutronic purity are disposed, usually in the` form of `a lattice arrangement of spheres or rods, in a neutron moderator such as graphite, beryllium or heavy water of high neutronic purity, preferably surrounded by a neutron reflector. Neutron absorption in the U238 content of the natural uranium during the reaction leads to the production of the transuranic isotope 94239, known as plutonium (symbol Pu), which is iissionablerin much the same manner as U235. Pu239, or 94239 is formed in neutronio reactors utilizing natural uranium in accordance with the following process:

kv. kand 270 kv., about l converted to electrons.

A small portion of the 94239 produced may also 4be changed to 94240 by absorption of neutrons. The Vneutronic reactors referred to above may be called isotope converters in that one iissonablerisotope is formed (94239) as another fissioriable isotope (U25) is ued up'. However, this conversion is not complete, andthe natural uranium,` which acts to supply both the react`ofn isotope (U235) and the absorption isotope (Um), .Will contain two different iissionable isotopes after the' v reactor hasl been started. Certain presently known uraniumigraphite"reactors have been found to have a conversion factor of .78, Uf35 to 94239. However, it rhy be desirable to form'other iissionable' isotopes in quantity such as, for example,U233.

The term neutronic purfty as used heren has no necessary relation to chemical purity and merely refers to the absence of foreign material having the charac- 4teristic of relatively great neutron absorption.. other words, aV substance may be said tohave highy neutronic purity, if the total amount of other material therelnris trons and is thus ineffective to poison the nuclear `fission chain reaction. v

It has been found that by so designing the reactor as toprovi'de a relativelyv high average neutron velocity 'of 'above'about 0.3 e.v., absorption of neutronsby such impurities,issubstantially reduced inasmuch as the capture cross sections thereof are greaterfor neutrons at energies'b'el'ow said value than for neutrons aty energies above said value. Thus it will be understood that al- Ythough there areA various velocity ranges, within which resonance absorption occurs in said impurities, maximumabsorption generally occurs vfor neutrons lbelow the aboveQi'neiitionedv value; and the loss of neutrons by absorption'in such impurities is substantially decreased by'4 designingthe reactor so that the average neutron 'en'g'yf'tl-re'rein is above said value.

Preferably the reactor comprises fissionable material and sucient neutron moderator to reduce the average neutron velocity to a value lower than about 1,000 e.v., and above about 0.3 e.v. Such an arrangement not only reduces neutron absorption by impurities as above discussed, but also reduces the amount of fissionable material necessary to sustain a chain reaction inasmuch as the fission cross section of such material is generally greater for neutrons below the value of 1,000 e.v. than for neutrons above said value.

To obtain conversion of one ssionable isotope to another in the most efficient manner, it is preferred to utilize a substantially neutronically pure fissonable isotope, such as 94239, for the neutronic reaction, and then form the new ssionable isotope U233 separately, from a substantially pure non-ssionable isotope, such es thorium:232 which in turn can be substantialy completzly` converted to the new fissionfbe isotope U23-3 fully recoverable in high purity and concentration from the thorium. Converters using the fissionable isotcpe in a liquid medium are disclosed and claimed in the copending application of Ohlinger, Weinberg, Wigner afd Young, Serial No. 628,322, filed November 13, 1945, now Patent 2,815,321, dated December 3, 1957.

The plutonium produced by neutronic reactors t's'ng natural uranium to support the reaction is useful for 'many purposes, but it has one outstand'ng advantage over, for example, the use of U235, as it exists in natural uranium. As plutonium is a different element from uranium it can be chemically removed from the irradiated natural uranium, and because of that fact can be obtained in substantially pure form and in high concentra- 'tions whereas U235 can only be obtained in high concentration or substantially pure form (as far as presently known) by the much more difficult process of isotrpe separation. U235 of high concentration, however, has been used to sustain a neutronic reaction.

In high concentrations of substantially pure form, plutonium can also be used, when properly combined with a neutron moderator, to sustain a slow neutron chain reaction in a neutronic reactor of relativey small size even where the neutron leakage is high. In other words, it can be used as an efficient source of large quantities of neutrons, and the neutrons thus'produced can be used to produce another fissionable isotope such as U233 can be formed by irradiating non-fissionablc thorium (90232) with slow neutrons according to the following process:

silam (average).

The foregoing and other objects and advantages of the invention will be more fully understood from a consideration of the following specification and the accomp'nying drawing, which is a diagrammatic illustrat'on part'ally in central vertical cross section of a structure embodying the invention.

Referring to the drawing, the reactor generally des'gnated 2 is disposed within a cylindrical tank or casing 4 of any suitable neutron permeable material such as aluminum, said tank comprising a wall 6 supporting a plurality of aluminum jacketed plutonium rods 8 wthin a body of fluid neutron moderator, such as ordinary water, circulated through the tank 4 by inlet pipes 10 and outlet pipes 12. The moderator uid thus serves the dual function of moderating the neutrons within the reactor and of removing the heat of nuclear reaction therewithin.

The tank 4 is supported within a tank or container 14 of neutron permeable material such as aluminum, said tank being preferably filled to the level indicated at A with a layer of neutron moderator such as beryllium, preferably compressed into a plurality of pelfes or pebbles. From level A to the level indicated at B, the tank 14 is filled with thorium containing material such as pebbles or pellets of compressed ThO2, and above the level B, the tank is preferably filled with another layer of neutron moderator such as the above-mentioned BeO pebbles. The material within the tank 14 is loaded into the top thereof and is preferably unloaded by means of a door 15 hinged to the tank 14 adjacent the lower open extremity thereof, said door being normally maintained in a closed position by means of a latch 17.

The tank 14 is supported bya wall 16 of a vault generally designated 18, preferably formed of a material such as concrete adapted to absorb biologically harmful emanations, such as neutrons and alpha, beta and gamma rays emanating from the reactor 2. The vault is closed by a cover 20 of any suitable material such as alternate layers of iron and Bakelite adapted to absorb the above-mentioned emanations. It will be understood that the vault 18 and cover 20 form a biological shield around the reactor 2 to protect operating personnel and equipment.

Preferably a cylindrical reflector 22 of neutron moderator such as Be or BeO is dsposed around the tank 14 and defines with the moderator pebbles therewithn a neutron reflector substantially enclosing the reactor 2 and associated thorium containing material, thus minimizing leakage of neutrons from the system and affording a maximum conversion rate at which the thorium is converted to U233 as above described.

It will be understood by those skilled in the art that the nuclear fission chain reaction within the reactor 2 may be controlled by regulating the level of the fluid moderator therein, or by any other suitabe means such as a control rod (not shown) of neutron absorber movable into and out of the reactor to vary the neutron rel production ratio thereof.

A reactor of the above-described design may be Constructed to operate at an average neutron energy above about 0.3 e.v. by providing not more than about 5 molecules of water to each atom of fissionable plutonium. Such a structure in the form of a cylinder may be constructed with a radius of about 19.41 centfmeters, a height of about 35.9 centimeters, and a volume of about 42.5 liters. Approximately 176 rods each having a height of about 35.9 centimeters and a diameter of about l centimeter are required to provide a total mass of 99.4 kilograms of plutonium, the rods being spaced about 2.59 centimeters from center to center, thus affording a structure in which the water moderator and ssionable plutonium are in the above-mentioned atomic ratio of the order of 5 to l. The rods are preferably spaced from the water by aluminum coatings about 1 millimeter in thickness. It will be understood that in such a reactor the average neutron energy may be increased by reducing the ratio of water to plutonium, thus necessitating the use of a greater total mass of plutonium inasmuch as the fission cross section thereof tends to decrease as the neutron energy increases, subject to the above-mentioned resonance ranges.

It will be understood that the above-described embodiment of the invention is by way of illustration and not limitation inasmuch as other embodiments of the invention will be readily apparent to those skilled in the art without departing from the spirit of the invention or the scope of the claims.

`What is claimed is:

1. A neutronic reactor active portion capable of op- References Cited in the le of this patent FOREIGN PATENTS Australia May 2, 1940 Switzerland Oct. 2, 1944 OTHER REFERENCES Smyth: Atomic Energy for Military Purposes, page 20, August 1945.

Smyth: Atomic Energy for Military Purposes, page centimeters from center to center and having thereon 10 45, August 1945.

aluminum coatings of 1 millimeter thickness.

Kelly et al.: Physical Review, 73, 1135-9 (1948). 

1. A NEUTRONIC REACTOR ACTIVE PORTION CAPABLE OF OPERATING AT AN ENERGY LEVEL OF 0.5 TO 1000 E.V. COMPRISING DISCRETE BODIES OF PU239 DISPOSED IN A BODY OF WATER WHICH CONTAINS NOT MORE THAN 5 MOLECULES OF WATER TO ONE ATOM OF PLUTONIUM, THE TOTAL AMOUNT OF PU239 BEING SUFFICIENT TO SUTAIN A CHAIN REACTION. 